Question

what is formed by two header joists and two end joists?
the rim joist
the floor deck
the box frame
the perimeter unit

Explanation:

In construction, a box frame is formed by two header joists and two end joists. The rim joist is a single member around the perimeter. The floor deck is the surface covering. The perimeter unit is not a standard term in this context.

Answer:

the box frame